The present invention relates to a method for making electrolytic capacitors and especially flexible and integrated electrolytic capacitors and capacitors for dynamic RAM (DRAM) applications, and in particular those having a high surface area anode.
An electrolytic capacitor is a capacitor in which one plate is metallic and the other plate is an electrolyte. Intervening between the two plates is a dielectric consisting of a surface oxide coating (e.g. of aluminum oxide) on the metal plate; it is known that the electrostatic capacity of such capacitors may be increased by including a complex oxide such as barium titanate or barium lanthanate with the oxide coating. In this connection, see e.g., Capacitors, Donald M. Trotter, Jr., Scientific American July 1988, pages 58-63, and JP 63304613. Conventionally, the metal plate on which the dielectric coating is formed is referred to as the anode. The term “anode” is used herein to refer both to the metal plate itself and to the combination of the metal plate with the dielectric coating. It will be clear from the context which meaning of “anode” is intended. A transition between ionic conduction in the electrolyte and electronic conduction in an external circuit is provided by a second metal plate, which is referred to herein as the cathode. The anode and the cathode are referred to herein collectively as electrodes. As will be seen from the description of the invention herein, the “plates” are in fact thin films.
Typically, the metal of the electrodes is a valve metal, i.e., a metal which, when oxidized, allows current to pass if used as a cathode, but opposes the flow of current if used as an anode.
As is the case with capacitors generally, the capacitance of an electrolytic capacitor is proportional to the surface areas of its two plates. Conventionally the surface areas of the electrodes are increased by etching, but in the case of thin film electrodes it is questionable whether they are capable of being etched sufficiently to afford the desired increase of surface area, while at the same time maintaining the mechanical integrity of the electrodes.
Vacuum deposition for increasing the surface areas of capacitor electrodes is known. Thus, Kakinoki, et al., in U.S. Pat. No. 4,970,626, describe vacuum deposition of titanium on aluminum foil, at an angle, to produce a titanium surface with a rough, columnar structure. Disadvantages of this method include the large costs of working with relatively thick layers of titanium.
On the other hand, Ohtuka et al. in U.S. Pat. No. 4,763,229 make an aluminum capacitor plate including a sponge-like layer formed by chemical or electrochemical etching, overplated by vacuum deposition of titanium particles. A disadvantage of this method is the additional cost of working with two systems: first etching in electrochemical baths and then sputtering in a vacuum system.
Drake, in U.S. Pat. No. 4,309,810, also teaches vacuum deposition of a metal vapor at a low angle onto a foil substrate, and presents an example of the deposition of aluminum on aluminum to give a columnar structure; however, the product has been found to be too brittle for use in electrolytic capacitors. Neumann et al., in German Patent No. 4,127,743, describe vacuum deposition of aluminum on aluminum in a low pressure oxygen atmosphere to give a surface structure of columns of aluminum separated by aluminum oxide. Allegret et al., in U.S. Pat. Nos. 5,431,971 and 5,482,743, also describe co-deposition, under a low pressure oxidizing atmosphere, of a mixture of aluminum and aluminum oxide. Such mixed Al/Al2O3 surfaces are more robust mechanically than pure aluminum surfaces, but electrolytic capacitors incorporating them are known to have relatively high resistive losses and relatively low stability over time. In addition, the presence of both aluminum and large quantities of aluminum oxide in the surface of the foil makes subsequent treatments such as conventional chemical or electrochemical stabilization, and structure coarsening by annealing, both difficult and less effective.
Bolz et al., in U.S. Pat. No. 5,571,158, describe a stimulation electrode having a porous surface coating whose active surface area is significantly greater than the surface area defined by the geometric shape of the electrode.
Having formed a metal electrode with high surface area, if the electrode is to be used as an anode, its surface must be oxidized. Conventionally, this is done by electrolytic anodization, in which the electrode is used as an anode in an electrochemical cell. Recent patents in this art include U.S. Pat. Nos. 4,537,665 and 4,582,574 to Nguyen et al., and U.S. Pat. No. 5,643,432 to Qiu. The thicker the oxide layer, the lower the capacitance of the electrolytic capacitor, but the higher the working voltage of the dielectric. For high voltage (upwards of 100V) applications, the dielectric layer is relatively thick, and tends to bridge over fine surface features, reducing the effective surface area of the anode.
Two other phenomena tend to reduce the effective surface areas of anodes made by electrolytic anodization. One is that in the course of the anodization process, oxygen and hydroxide ions migrate from the metal-dielectric interface into the metal, while metal ions migrate from the metal-dielectric interface into the dielectric. The other is that sharp points on the metal surface are characterized by high local electric fields, which accelerate the electrolytic process. Both of these phenomena tend to smooth out irregularities in the metal-dielectric interface.
It is also known to use TCNQ complexes as solid electrolytes, and to coat them on electrolytically anodized metal surfaces by vapor deposition techniques (see e.g., JP 6036966, and JP 62094912, -3 and -4). In JP 63069149, polyethylene oxide and a lithium salt are vapor deposited on a substrate for use as a solid electrolyte.
However, in general terms a solid electrolyte need not be TCNQ and it need not be vapor deposited. In JP 7183172 and JP 2241014, capacitors are made by dipping capacitor elements in molten TCNQ complex salt, or in a solution thereof, respectively. In U.S. Pat. No. 4,090,231 (Millard et al.), a capacitor is made by sequentially screen printing on a valve metal (e.g. Ta) substrate, a mixture of valve metal (e.g. Ta) powder plus binder, sintering the printed layer, forming a dielectric oxide, applying a solid manganese dioxide electrolyte layer, and forming a counter electrode. In JP 63105962 a thin film of solid electrolyte, said to have superior ionic conductivity, and selected from RbCl, RbI, KCl, KI, CuCl and CuI, is deposited on a copper plate by thermal evaporation.
It is also known to form a plurality of capacitors on a single substrate, see e.g. the above-mentioned Millard et al. patent, in which a high surface area is provided by a porous pad with a rough outer surface of valve metal being sinter-bonded to valve metal substrate t-face, as well as U.S. Pat. No. 5,357,399 (Salisbury), in which a solid electrolyte film is connected electrically and mechanically to a metallic member parallel to and substantially coextensive with a metallic substrate, and any voids in the capacitor may contain injected insulating material. In Salisbury, a large capacitance is provided as a result of a large surface area of metal within a sintered porous mass; a disadvantage of this technology is the complicated method of creating large surface area and more especially the high temperature involved, which makes it incompatible with other integrated passive components technologies.
In recent years, integrated passive devices such as capacitors have assumed an ever-increasing importance, particularly in order to make the most advantageous use of small areas, where, by contrast, conventional use of numerous discrete passive components is at a distinct disadvantage (see e.g., “Packaging Technology: Integrated Passive Devices”, A. Chalaka, Passive Component Industry, March/April 2000, Vol. 2, No. 2, 14-16).
In U.S. Pat. No. 5,851,871 (Re), there is described an integrated capacitor typically comprising a “sandwich” formed by two layers of suitably doped polycrystalline silicon separated by a film or thin layer of silicon oxide; one of the silicon layers can be replaced by aluminum.
In U.S. Pat. No. 5,589,416 (Chittipeddi), there is described an integrated capacitor comprising a polysilicon/silicon dioxide/TiN combination.
U.S. Pat. No. 4,453,199 to Ritchie et al. describes a method of forming, by vapor deposition or the like, discrete plain (i.e. not rough) electrode areas on insulating substrates, and depositing a dielectric layer over the electrodes and the areas between the electrodes.
To the best of the present inventors' knowledge, a high specific surface area vapor deposited valve metal film has not been previously suggested as the anode in an integrated electrolytic capacitor, nor when this is combined with a vapor-deposited film of solid electrolyte, nor when combined with an oxide layer produced by plasma anodizing and a vapor-deposited film of solid electrolyte.
Moreover, to the best of the inventors' knowledge, the combination of a vapor deposited anode with a vapor deposited electrolyte has not been previously suggested even as a basis for standard wound (i.e. not integrated) electrolytic capacitors, still less when combined with an oxide layer produced by plasma anodizing.
There is a widely recognized need for improved and economically viable methods of manufacturing high capacitance compatible with other thin film technologies for integrated passive devices, and for mechanically robust integrated electrolytic capacitors.
There is also a widely recognized need for creating standard wound electrolytic capacitors by a method involving a minimum number of technological operations, or in which all or almost all layers are made by vapor deposition.
The present invention is believed to make a significant contribution in fulfilling these needs.
The entire contents of the patents and other publications mentioned herein, including issued patents corresponding to U.S. Ser. No. 09/033,664, are deemed to be incorporated by reference in the present patent application.